Despite data from clinical trials showing an increased risk of breast cancer and lack of overall benefit for primary prevention with hormone therapy (HT), there is still significant usage of HT by clinicians and women for treatment of moderate-to-severe systemic menopausal symptoms. Due to the possible increased risks with these preparations, current clinical recommendations advocate utilizing the lowest dose for the shortest possible duration, but average duration is often for 2-4 years. However, the effects of commonly used low-dose HT regimens on breast density and breast cancer risk are unknown. Recent data from intervention trials support the long standing hypothesis that hormonal effects on breast cancer risk are mediated through breast density. The Kronos Early Estrogen Prevention Study (KEEPS) is an ongoing, already- funded randomized clinical trial with a primary goal of examining the effects of 4 years of low- dose transdermal versus oral estrogen combined with cyclic micronized progesterone on progression of subclinical atherosclerosis in recently menopausal women. The proposed ancillary study will efficiently examine the effects of randomized low-dose HT on key breast cancer surrogate endpoints: mammographic density and rates of abnormal mammograms. In KEEPS, a total of 720 women were randomized to oral conjugated equine estrogens (0.45 mg/d), transdermal 17-beta estradiol (50 mcg/d), or placebo, with cyclic micronized progesterone (200 mg for 12 days) for active hormonal therapy groups. All participants were required to have mammography prior to study entry as well as yearly during follow-up; blood samples also were collected at these time points. Randomization was completed in 2008. In this ancillary study, digital mammograms (available on 510 participants) from baseline, year 1 and year 3 will be obtained and processed centrally to calculate mammographic density. We will determine if these two low-dose HT regimens are associated with change in mammographic density. We will also examine if baseline mammographic density or circulating biomarkers including estradiol, estrone, and estrone sulfate, modify or mediate changes in breast density with HT. In addition, the effects of hormonal regimens on the rates of abnormal mammogram and biopsy will be determined. By determining the effects of low-dose combined HT on the breast, the proposed study will add timely and important information about the estimated balance of risks and benefits associated with low-dose HT and will help guide future research.